R gunn



R. GUNN Re. 21,372

APPARATUS FOR THE DETECTION OF DISPLACEMENTS Feb. 27, 1940.

Original Filed Dec. .4, 1936 3 Sheets-Sheet 1 m mu 6 wx IO 8'I'I'L'I'I'I mi mil mvznron Ross Gunn (LMH ATTORNE Y Feb. 27, 1940. GUNNRe. 21,372

' PFARATUS FOR THE DETECTION OF DISPLACBNBNTS OriginaLI-iled Dec. 4,1936 :s Shoots-Sheet 3 EVZZN INVENTOR Ross Gunn' ATTORNEY Reiuued Feb.27, 1940 UNITED STATES PATENT OFFICE APPARATUS FOR THE DETECTION OFDISPLAOEMENTS Itoss Gunn, Washington, D. 0.

8 Claims.

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) My invention relates to anapparatus for de tecting displacements and more particularly to asimple, rugged and eilective electrical apparatus for the remote andsubstantially instantaneous detection or measurement of mechanicaldisplacements which maybe either large or small in magnitude.

In carrying out my invention, I employ a space discharge device whichincludes at least a cath- I ode and a movable anode and provide meansresponsive to the displacement to be detected or measured for impartingmovement to the said anode. Any movement imparted to the anode resultsin a change in the average internal'resistance between the anode andcathode with at tendant change in the space current through the spacedischarge device. By connecting the space discharge device in a propercircuit and providing suitable indicating or recording means, adisplacement of any desired character or magnitude may be detected ormeasured. Various types of novel space discharge devices and circuitsmay be advantageously employed in carrying my invention into efiect allof which will be pointed out more in detail hereinafter.

The apparatus of my invention is capable of detecting or measuring highfrequency periodic or non-periodic mechanical displacements,

other structures; for detecting or recording earthquakes, either naturalor such as are produceduas incidental to geo-physical exploration; fordetecting or measuring elastic deformation such, for example, as wouldoccur in a ships structure or a propeller shaft; for detecting ormeasuring angular displacements; and for detecting or measuringinstantaneous liquid or gaseous pressures. Many other uses will readilyoccur to those skilled in the a'rt..

With the foregoing preliminary discussion in view, it is an object of myinvention to providea simple, rugged and effective electrical apparatusfor detecting or measuring periodic or. nonperiodic mechanicaldisplacements of either large or small magnitude.

It is another object of my invention to provide -an electrical apparatusfor detecting or measuring mechanical displacements wherein any changein the value of an electro-motive force incorporated in a balancedelectrical circuit of the apparatus or any change in the electronemissivity of a cathode of a space discharge device incorporated in alike circuit and forming part of the apparatus will not affect anyindication of the indicating means.

It is another and further object of my invention to provide anelectrical apparatus for detecting or measuring angular displacements.

It is'another and still further object of my invention to provide newand novel space discharge 1! devices for incorporation in my detectingor measuring apparatus but which, however, are susceptible of beingadvantageously used in other environments.

Other objects and many of the attendant advantages of this inventionwillbe readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings, wherein:

Fig, 1 discloses one embodiment of my invention;

Fig. 2 discloses an alternative embodiment of my invention employing adifierent space discharge device and circuit arrangement; .30

Fig. 3 depicts a further modification of my invention dlfiering overthat of Fig. 2 principally in Fig. 6 is a sectional view taken on theline G6,

Fig. 5;

Fig. '7 shows another modification of my apparatus employing twocathodes in the space discharge device thereof and an amplifier foramplifying any voltage diiiferential of the bridge cir- 4,5 cult;

Figs; 8 and 9 represent different embodiments of my invention whereinthe displacement of the movable electrode or electrodes of the spacedischarge device is controlled by suitable external magnetic means; andI 4 Fig. 10 depicts a modification of my invention suitable for thedetection or measurement of fluid pressure. 1

Before proceeding with a detailed description oi. the variousembodiments of my invention it is deemed advisable at this point in theinterest of clarity and in order to avoid needless repetition in thesubsequent description to make certain general remarks concerning thespace discharge devices and circuits employed in my apparatus. Aspointed out hereinbefore, each space discharge device includes at leastone cathode which serves as a source of electrons. A cursory examinationof the drawings will show that all cathodes have been depicted thereinas of the thermionic type, the heating thereof to insure electronemission being accomplished either directly or indirectly by a suitablesource of electro-motive force. I

wish to emphasize in this connection, however,

that I do not desire to be restricted to this type of cathode sincecathodes which emit electrons due to bombardment by rapidly moving ions,electrons, or metastable atoms and are said to be secondarily emissive,or photo-electric cathodes, or cold or non-thermionic cathodes whichspontaneously emit electrons due to treatment thereof by a radioactivesubstance, all will serve the purpose of my invention equally well. Asfor the thermionic cathode, it may be fabricated of a pure metal, beoxide-coated, be of a metal provided with an adsorbed monatomic film ofone of the electro-positive metals or be of any other type known to theart. The envelopes of my space discharge devices which are fashionedfrom any of the materials known to the prior art and serve to enclosethe electrodes may be evacuated to'produce a high vacuum and thus insurea substantially pure electron discharge or may alternatively be providedwith a suitable filling measured serve to impart movement to at leastone of the said three electrodes to cause simultaneously thereamong adecrease in electrical resistance'between the first and secondelectrodes and an increase in electrical resistance between the firstand third electrodes, thus causing an unbalancing of the bridge circuitwhich is indicated by a suitable indicating or recording device. It is,of course, apparent that the indicating device may besuitably calibratedto indicate not only the magnitude of the displacement but also itsdirection.

The basic principle underlying my invention I may be best appreciated byconsidering the embodiment disclosed in Fig. 1 of the drawings.

There is shown depicted in this figure an envelope i of a spacedischarge device which includes as part thereof a flexible bellows-likeme tallic or glass extension 2 to the end of which is rigidly securedthe anode 3 by means of the lead-in wire 4. A cathode 5, which may be ofa thermionic type, is heated to an electron emissive temperature by thesource of electro-motive force' 8. The plate circuit of the spacedischarge device includes the source of electromotive force I and anindicating device I which may be. a milliammeter. jlhe force, pressureor displacement to be detected is represented schematically by the arrow9. A small spring and anvil ii) are arranged to return the extension 2of the space discharge device to its initial and desired position ofrest upon removal of any actuating force or pressure. It is evident thatthe mounting of the anode 3 is such that the application of a force ordisplacement 9 as shown 'will change the mean spacing between the anode3 and cathode 5, this displacement of the electrodes being made possibleby reason of the flexible bellows-like extension 2.

It is well-known in the vacuum tube art that with a given platepotential the space current density varies in a non-linear manner withthe electrode spacing and increases rapidly as the spacing between theanode and cathode structures is reduced. This fact was first establishedby Child and is known as Childs law. Thus, the average internalresistance between the anode 3 and cathode 5 is a definite andreproducible function of the spacing between these electrodes, whichspacing is controlled by displacing the end of the flexible extension 2by a force or pressure 9. It is therefore evident that any change in.

spacing between the electrodes 3 and 5 will result in a change inspace-current, the indicating device 8 reading more or less dependingupon whether the spacing between the electrodes is decreased orincreased. Hence, any departure from a predetermined and initial readingor reference point of the milliammeter serves to detect a displacement,the magnitude of which is easily ascertainable by suitable calibrationof the milliammeter or indicating means 8.

While the apparatus of Fig. 1 is an entirely operative and usefulstructure it does, nevertheless, possess certain inherent disadvantages.If, for example, the emissivity of the cathode 5 is in any mannerimpaired or the value of the electromotive force of either sources 6 orI changes, this will be evidenced by first order changes in the plate.current indicated'by the milliammeter I. If, then, the indicating meanshas been previously calibrated for a fixed electron emissivity andflxed'values of electromotive forces, it is evident that the indicatingmeans 8 will now give inaccurate and false readings because of a changedreference point. In order to minimize any such difliculty as might beencountered in using the apparatus of Fig. 1, I provide different tubestructures and circuit arrangements as will be pointed out hereinafter.

In Fig. 2 of the drawings there is shown a space discharge device andcircuit arrangement which minimize the difllculties that may be encountered with-the apparatus of Fig. 1. In this figure Ii denotes theenvelope of the space discharge device which like that of'Flg. 1 has alone end thereof a flexible metallic or 'glass bellows-like extension iito which are secured and insulated from each other the two anodes I! andI4 supported by their respective lead-in wire: IS, IS, The cathode l'lwhich may be of a thermionic type is heated'to an electron emissivetemperature by the battery It. The insulated leadin wires l5 and ii areconnected in a bridg circuit which includes the two bfla'ncing resistancesl9 and Ill. It is thus seen that $111 bridge circuit includesfour arms or branches two of which are constituted by the resistancebetween the anodeil, cathode H, and anode l4 cathode IT with theresistances l8 and hereto fore mentioned forming the remaining tw'branches. The indicating device 2! which ma;

Me milliammeter is placed across one diagonal of the bridge and theplate potential or source oi electromotive force 22 connected across theremaining diagonal. A small spring and anvil 28 again serve to returnthe extension or projection l2 thespace discharge device to its initialv and desired position of rest upon the removal of any actuating force.Ii. now the end or the extension or projection I2 is displaced in anymanner, such as for example by the indicated micrometer 24, the anode l2will move toward the cathode ll while simutaneously the anode l4 willmove away from the cathode, this movement of the electrodes occurringdue to the fact that the anodes l8 and I4 are rigidly secured to theendofthe projection or extension l2 by means of the lead-in wires l5 andI8. The former electrode movement tends to decrease the internalresistance between the anode l2 and cathode l1 and the latter has a.tendency to increase the resistance between the anode l4 and cathode ll.Ii, then, the bridge be balanced by a suitable adjustment oi thevariable resistances l8 and 2|] prlor to the application of a force bythe micrometer 24, the bridge will now be unbalanced in both its arms bythe displacement oi the micrometer. It is at once evident that anyunbalancing of the bridge circuit will be indicated by the milliammeter2i and that the direction of the indication of the milliammeter 2| willindicate the direction of the displacement. 'The apparatus of Fig. 2possesses important advantages over that disclosed in Fig. 1. Forexample, ii the emission oi the cathode i1 is in any manner impaired orif the values of the electromotive forces l8 and 22 change, thesechanges or variations in emissivity and voltage will aflect the platecircuits in the bridge equally and to the first order at least. Thesevariations or changes. however, will not be indicated by themilliammeter 2| provided the bridge is balanced as indicated in Fig. 2of the drawings. Hence, the

electrical balance or zero reierence point willrunain fixed undervarying conditions of plate plate voltage and emissivity. This is adistinct advantage and improvement over the apparatus 01 Fig. l in thatany correction to the meter reading can now be easily made when basedupon a fixed reference point; whereas corrections to the meter in theapparatus of Fig. 1 are extremely dimcult because of its uncertainreference point occasioned by any change in emissivity or voltage. It isthus clear that the apparatus of Fig. 2 provides a very rugged andpracticable structure for indicating or measuring displacements.

The apparatus shown in Fig. 3 is a variation of that shown in Fig. 2. Inthis figure the envelope 25 of the space discharge device is provided atone end thereof with a flexible metal seal 28 sealed to it inany mannerknown to the prior art, the said seal 24 having secured thereto thetubular metallic member 21 supporting the anode leads 28 and 28. Theanode lead 28, as shown, supports the anode 88 and is hermeticallysealed to and insulated from the tubular member 21 by means of theinsulating plug 8|. The metallic' tubular member 21 besidesserving to impart movement to the anodes 88 and ,82 serves also as a currentconductor. The cathode 38, which may be of a thermionic type, is heatedto an electron emissive temperature by the battery 34 and is surroundedby a grid 38 which is connected to a suitable source of biasing voltage88.

The space discharge device 28 as in the preceding figure is connected ina bridge circuit to form two branches thereof, the remaining twobranches being constituted by the variable resistances 81 and 38. Anyconvenient source of electr'o-motive force 39 serves to supply thecurrent for energizing the bridge circuit. The tubular member 21 withthe anodes 38 and 82 rigidly secured thereto is moved by any means suchas the indicated micrometer 48. An oscillographic element 4! is shown inthe bridge circuit to emphasize the fact that periodic or rapiddisplacements of the small projecting tube may be indicated and recordedby photographic or other methods. The essential diiierence oi theapparatus of Fig, 3 over that of Fig. 2 resides in the employment of agrid structure 35 and a biasing battery 36. This arrangement can be madesomewhat more sensitive than that of Fig. 2 and balancing of the bridgeis somewhat more convenient. The apparatus of Fig. '3, however, alsoemploys the fundamental principle of a change in plate resistanceattending the simultaneous movement of the anodes 38 and 32 as a resultof the application of a. pressure or force to the tubular member 21. I

Fig. 4 shows another modification of my apparatus wherein 42 denotes theenvelope enclosing the anodes 43, 44 and the thermionic cathode 45 whichis heated to an electron emissive temperature by the battery 46. Atubular member 41 supports the anodes 43 and 44 for movement within theenvelope and is identical in construction and mounting with that shownin Fig. 3 of the drawings. It is to be clearly understood that the spacedischarge device of Fig. 4 is identical in construction and operationwith that 01' Fig. 3 except that the grid has been omitted. Adiiierential galvanometer 48 with an appropriate balancing resistance 49is connected in the plate or bridge circuit of the space dischargedevice, a source of electromotive force 50 serving to energize thecircuit. Thewindings 5| and 52 of the differential galvanometer are, oicourse, connected so that they annul each other and if the currentincreases in one coil and decreases in the other, the deflection will beincreased due.

to both causes. The balancing resistance 49 connected across the coil 5|permits the employment of the differential galvanometer as a nullinstrument. Again, a spring and anvil are schematically represented at53 to return the tubular member 41 to its initial and desired positionof rest upon the removal of any actuating i'orce. Anvils 54 and 55 and atest object 58 suggest one use of the apparatus which is applicable tothe modifications previously described and to be described hereinafter.Thus, for example, it the test object 56 is a standard test piece and isintroduced between the anvils 54 and 58 as shown the bridge may bebalanced so thatthe galvanometer 48 will read some predetermined value.Then as object 56 is removed and comparison objects are introduced inplace thereof, the galvanometer or indicator 48 will read more or lessthan the value established for the standard test piece, thus showingthat the object under test is greater or less in diameter than thestandard. The galvanometer' 48 is readily calibrated and standardizedwith the result that the exact amount of departure of any test piecefrom that of the standard can be easily and quickly determined.

Figs. 5 and 6 illustrate a still further modification of my apparatuswhich is especially designed for indicating or measuring angulardisplate structures GI] and GI are alsoeach made of a plurality ofconductors which parallel to those of the grid and substanti ly parallelto the cathode and are so arranged that when the conductors of the anode60 (see Fig. 6) are behind the grid conductors and thus substantiallyfully shielded from the cathode, the conductors of the anode 6| aresubstantially fully exposed to the cathode. The envelope of the spacedischarge device includes an insulating portion 62 to which is sealed ametallic portion "having a part thereof terminating at 64 which isfiexible and to which twisting or torsional movements may be imparted.Rigidly secured to the part or extension 54 is a tubular metallic member65' in which the lead-in wires of the anodes l and Bi are 'insulatinglysupported and hermetically sealed by the insulating plug 66. It

. is evident that any angular movement of the extension 64 will resultin rotary or angular movement of the anodes 60 and BI. A suitable sourceof electromotive force 61 serves to bias the grid while the remainingelectrodes of the space discharge device are connected ina bridgecircuit which includes the variable resistances 68 and 69 and thevoltage source Ill for energizin the same. As in the previousembodiments of my invention a suitable indicating device H is providedfor detecting any differential voltage in the bridge circuit. The mannerof operation of the apparatus in Fig. is believed to be clear. Thus itany torsional movement is imparted to the flexible extension 64 by anymeans whatsoever rotary movement of the anodes ill and BI will result.Assuming that the initial positions of the anodes 60 and GI are shown inFig. 6 if the movement is in the direction indicated by the arrow theconductors of the anode 60 will be exposed more and more to the cathode58 while simultaneously the conductors of the anode 6| will be movedprogressively from their exposed positions to the cathode to positionswhere they are shielded by the grid conductors. If the bridge bebalanced prior to the initiation of any movement it is clear .that theangular displacement of the anode 60 will result in an increased currentflowthereto while the current to the anode II is decreased. Sincemovement of the anodes causes a variation '-in the tube resistances withattendant variation oi currents in the bridge circuit it is evident thatthe bridge is now unbalanced and that any difierential voltage thuscaused is indicated by the indicating device II. In this manner it ispossible to indicate or measure angular displacements.

In the modification of Fig. '1, 12 indicates the envelope of the spacedischarge device to which is sealed the flexible metallic cap I! formovably mounting the anode 14. The anode as shown is interposed betweentwo cathodes l5 and I6 01' the indirectly heated type which are heatedto an electron emissive temperature by the resistance units 11 and Itenergized by the battery It. A small spring and anvil 80 serve to returnthe anode to its initial and desiredposition of rest, movement beingimparted to the anode, for example, by the test piece Bi which is showninterposed between the anvil 82 and micrometer 83.

The space discharge device of Fig. 'I has certain advantages over thosepreviously described in In this embodiment 51 designates that it is notnecessary to insulate two moving lead-in wires for the anodesand as inthe previous embodiments is connected in a bridge circuit which includesthe variable resistances 84 and 85 and a suitable source ofelectro-motive force 86.

over those previously described in that any dif-. ierential voltageproduced in one diagonal of the bridge circuit is amplified, this beingaccomplished by connected parts of the bridge circuit as shown to grid86 and an indirectly heated cathode 81 of an amplifier tube 88. .Thisamplifier tube in turn forms one branch oi a second or auxiliary bridgecircuit, the remaining three branches of which are constituted by thevariable resistances 89, BI! and 9| and energized by a suitable source0! electromotive force 92; This scheme insures amplification of anydifferential voltage in one diagonal oi the first bridge circcuit whichis indicated or recorded by any proper indicating means such as, forexample, an oscilograph 93. It is to .be emphasized that thisarrangement for amplifying any differential voltage may be employed inany other modification of my invention and that any convenient number ofstages may be employed.

A further modification of my invention is shown in Fig. 8 in which theanodes It and I! are made 0'! a magnetic material which can bemagnetized by induction or maybe permanent magnets with poles asindicated. The anodes are secured to relatively flexible lead-in wires96 and 81, a cathode 98, which may be of a thermionic type. beinginterposed between the said anodes. The electrodes oi! the spacedischarge device are connected in a bridge circuit to form .two branchesthereof, the remaining two branches being formed by the variableresistances l8 and Ill and encrgized by a suitable source ofelectro-motive force Ill. As in the previous modifications an'indicating device I02 serves to indicate or measure any displacement ofthe anodes M and 5 with respect to the cathode 98. The position of theanodes within the space discharge device is controlled by any suitableexternal magnetic means such as, for example, a permanent magnet I". Theposition of the magnet I03 is controlled by The apparatus depicted inFig. 7, however, diflers the displacement which it is desired to detector measure, it being apparent that the magnet may be moved in a, desiredmanner. Thus, for examf pie, rotation of the magnet I about its axis a!symmetry will produce a displacement of the magnetic anodes within thetube in such a way that angular displacements may be readily indicatedon the milliammeter or ibdicating device Fig. 9 illustrates another formwhich my apps.- ratus may take. In this embodiment two thermioniccathodes are employed, one 01' which is designated in general by thenumeral I and made in two portions I04 and Ill", the remaining cathodeI05 being similarly constructed and consisting of the parts It! and IN".The cathodes I and I" are heated to an electron emissive temperature bythe respective sources of electro-motive force I" and ill. The anode isshown at I08 as a magneti'zedbody and is interposed and mounted forrotation between the cathodes. The space discharge device as in thepreis indicated by a suitable indicating device II2. As in themodification of Fig. 8 the position of the anode I08 is controlled byany suitable external magnetic means such as, for example, the magnetII3, the position of the said anode with re- II! and H6 and transverselyof which there extend two flexible partitions H1, H8 which may be ofmetal to thus provide upper and lower chambers II9, I20 and anintermediate hermetically sealed chamber I2I. Anodes I22 and I2! areelectrically connected to and supported by the respective diaphragms orpartitions H1, H8 a thermionic cathode I24 being interposed between theanodes for cooperation therewith. As in many of the precedingembodiments the electrodes within thehermetically sealed chamber I2I areconnected to form two branches of a bridge circuit of which theremaining two branches are constituted bythe variable resistances I25and I26. Any suitable indicating device I2I serves to indicate anydiiferential voltage in the bridge circuit which is energized by thesource of electro motive force I28. The chambers Ill and I2Il serve asreceptacles for the fluid the pressure of which is to control thedisplacements of the diaphragms III and Ill. It is evident-that anypressure applied to the diaphragm ill will cause a displacement thereofthus decreasing the spacing between the anode I22 and the cathode I24with attendant unbalancing of the bridge circuit as evidenced by anindication of the indicating means I21. The indication of the indicatingmeans I2I will of course be proportional to the displacement of thediaphragm Ill. The diaphragm IIB which is electrically connected in thebridge circuit and to the anode I23 is introduced for two reasons. Itserves as a means for balancing out irregularities in emission of thecathode or any voltage variation in the sources of electro-motive forceunder balanced circuit conditions but is primarily intended to correctfor any changes in barometric pressure. As pointed out herelnbefore, ifthe dia-' 1 phragm II 'I is subjected to fluid pressure the displacementof the diaphragm will'be proportional to the pressure exerted upon it.If, for example, the apparatus is employed as a fuel gauge in aircraft,operations at high altitudes will reduce the displacement of thediaphragm II! and in order for the indicating device I2I to read theproper pressure-head the anode I23 must be displaced a distance equal tothe distance displaced by the anode I22 due to the increased altitude.To insure a displacement of the anode I23 and hence a correct andaccurate reading of the indicating device I21 the diaphragm H8 isprovided. It is clear that the apparatus of Fig. 10 may be used toindicate diflerential pressure.

All novel features of my system or apparatus as a whole are claimed inthis application, the features of novelty of my space discharge devices,however, being claimed in application Serial Number 114,111 filedconcurrently herewith.

According to the provision of the patent statutes I have set forth theprinciple and'mode of operation of my invention and have illustrated anddescribed what I now consider to represent its best embodiments.However, I desire to have it understood that within the scope of theappended claims the invention may be practiced otherwise than asspecifically illustrated and described.

The invention herein described and claimed may be used and/ormanufactured by or for the Government oi the United States of Americafor governmental purposes without the payment of any royalties thereonor therefor.

I claim:

1. An apparatus for detecting mechanical displacements comprising incombination an envelope enclosing at least three electrodes, at "leastone of which is electron emissive and at least one of which is movable,means responsive to the mechanical displacement to be detected forimparting movement to at least the movable electrode, and meansassociated withv said electrodes for indicating any detecteddisplacement.

2. An apparatus for detecting mechanical displacements comprising incombination an envelope enclosing two spaced anodes and an electronemissive cathode interposed therebetween, at least one of said anodesbeing mounted for movement relatively to the envelope, meansresponsiveto the mechanical displacement to be detected for impartingmovement to at least the movable anode, and means associated with saidelectrodes for indicating any detected displacement.

which are arranged'such that when'the conductors of one' anode aresubstantially, fully shielded'by the grid conductors ffom the cathodethe conductors of the other anode are substan-'- tially fully exposed tothe cathode, means responsive to the angular displacement to be detectedfor imparting rotational movement to said anodes to thereby vary thepositions of the anode conductors with respect to the grid conductors, abridge circuit in which the aforementioned cathode and anodes areconnected to form two branches thereof, a grid circuit for suitablybiasing the grid, and means for indicating any detected displacement.

5. An apparatus for detecting displacements comprising in combination anenvelope enclosing two spaced electron emissive cathodes and an anodeinterposed therebetween, means responsive to the displacement to bedetected for imtity comprising in combination an envelope ene closing atleast three electrodes, at least one of which is electron emissive andat least one of which is movable, means responsive to the mechanicaldisplacement to be converted for imparting movement to at least themovable electrode and a bridge circuit in which three electrodes areconnected to form two branches thereof. 1

7. An apparatus for converting a mechanical displacement into anappropriate electrical'quantity comprising in combination an envelopeene closing at least three electrodes, at least'one of which is electronemissive and at least one off which is'movable, means responsive to themechanlcal'displacement to be converted for imparting movement to atleast the movable electrode, a normally balanced circuit in which threeav electrodes are connected to form two branches thereof and meansassociated with said circuit for indicating a condition of unbalance.

8. An apparatus for converting a mechanical displacement into anappropriate electrical quantity comprising in combination an envelopeenclosing at least three electrodes, at least one of which is electronemissive and at least one of which is movable, means responsive to themechanical displacement to be converted for imparting movement to atleast the movable electrode; a bridge circuit and an electricalinstrument connected across one diagonal of the bridge circuit forenergization thereby, three electrodes being connected in the bridgecircuit to form two branches thereof and the remaining two branchesthereof being constituted by a plurality of balancing elements.

